Excitation-Contraction Coupling and Reflexes PDF

Excitation contraction coupling and reflexes Control of muscle innervation Sympathetic nervous system Chain of sympathetic ganglion Preganglionic neurons release acetylcholine...

Excitation contraction coupling and reflexes Control of muscle innervation Sympathetic nervous system Chain of sympathetic ganglion Preganglionic neurons release acetylcholine Postganglionic neurons release noradrenaline Sympathetic neurons located next to targets Participates in ‘fight-or-flight’ response By binding to different postsynaptic adrenoreceptors: Relaxes airways in lungs Inhibits digestion Accelerates heart rate Excitation contraction coupling and reflexes 1  Adrenaline used to treat patients in cardiac arrest (increases peripheral resistance & accelerates heart rate) close due to SNS being involved in fight or flight response Excitation contraction coupling and reflexes 2  Parasympathetic nervous system Releases acetylcholine Complements sympathetic nervous system, acts as a brake to the sympathetic system Controls actions that do not require an immediate reaction: digestion metabolic function regulates kidneys, liver etc. By binding to different postsynaptic acetylcholine receptors, results in: constriction of airways stimulates digestion slows heart rate Parasympathetic neurons travel large distances Aps propagate over long distances due to more housekeeping effect Excitation contraction coupling and reflexes 3  Parasympathetic division Somatic motor system Single function: innervate and command all skeletal muscles in the body Excitation contraction coupling and reflexes 4  Under voluntary control and generates behaviour The somatic motor system: CNS projection to spinal cord Lower motor neurons Directly command muscle contraction Skeletal muscle movements, initiate by ‘lower’ motor neurons Activated by local spinal cord circuits Each motor neuron innervates a single muscle Excitation contraction coupling and reflexes 5  Organisation of lower motor neurons Lower motor neurons are distributed within the ventral horn in a predictable way Spatial map of body musculature Neurons innervating axial muscles are medial to those innervating distal muscles Neurons innervating flexors are dorsal to those innervating extensor muscles Excitation contraction coupling and reflexes 6  The motor unit Alpha motor neurons directly trigger the generation of force by muscles Single motor neuron synapses with many muscle fibres to ensure the spread of the contractile force is even Excitation contraction coupling and reflexes 7  Small motor neurons innervate small muscles Large motor neurons innervate large muscles The alpha motor neuron and all the muscle fibres it innervates is classed as the motor unit The motor neuron pool Muscle contraction results from other individual and combined action of motor units Motor neuron pool - collection of alpha motor neurons that innervate a single muscle (e.g. biceps) This arrangement maintains normal muscle activity when damage to a single motor neuron occurs Excitation contraction coupling and reflexes 8 Excitation contraction coupling and reflexes 9  Structure of the neuromuscular junction Neurotransmitter release at the neuromuscular junction Muscle contraction initiated by ACh release and binding to postsynaptic ACh receptors 1 presynaptic action potential is sufficient to trigger 1 postsynaptic action potential in a muscle fibre Excitation contraction coupling and reflexes 10  Structure of a muscle fibre Sarcolemma: excitable cell membrane covering muscle fibre Myofibrils: contract in response to an action potential sweeping down the sarcolemma Sarcoplasmic reticulum: extensive intracellular sac that stores calcium ions itself is a calcium store Excitation contraction coupling and reflexes 11  Molecular basis of excitation-contraction coupling Excitation contraction coupling and reflexes 12  Muscle contraction: thin actin filaments slide along the thick myosin filaments Contraction coupling: Ca2+ binds to troponin, exposing myosin binding sites on actin Myosin binds actin, myosin head pivots, sliding actin down Myosin disengages at the expense of ATP Excitation-contraction coupling - muscle contraction 1. AP in the alpha motor neuron 2. Exocytosis of ACh 3. Postsynaptic depolarisation 4. Ca2+ release from sarcoplasmic reticulum 5. Sliding actin / myosin filaments 6. Muscle contraction Relaxation occurs when Ca2+ or ATP level reduce Excitation contraction coupling and reflexes 13  Single muscle twitches to sustained contraction Control of muscle contraction Alter the firing rate of motor neurons (1 action potential = twitch) (80Hz action potentials = full contraction) Recruit more motor units - increase contraction Excitation contraction coupling and reflexes 14  What is a reflex? An involuntary, nearly instantaneous movement in response to a stimulus (does not require the brain) Myotatic or stretch reflex: reciprocal innervation of flexors and extensors Knee-jerk reflex Excitation contraction coupling and reflexes 15  Spiking patterns of neurons during the knee-jerk reflex Crossed-extensor reflex Reflex pathways: Painful stimulus Excitation contraction coupling and reflexes 16  Activation of sensory (afferent) axons Activates excitatory interneurons Motor neurons (efferent) induce contraction But on its own this reflex would make you unstable Excitation contraction coupling and reflexes 17  Muscles don’t work separately - coordinated activity to ensure stability Excitation contraction coupling and reflexes 18 Excitation contraction coupling and reflexes 19